PHYSICAL RESEARCHES CONTINUED 73 



sequence of their atomic weights from Lithium, with its 

 lowest atomic weight 7, to Lead, with high atomic weight 

 205. He was surprised to find that the ' electric touch ' 

 exhibited a periodic change. When a substance exhibits 

 under electric radiation an increase of conducting power, 

 its sign of ' touch ' he distinguished as positive. This is 

 the strong characteristic of the ' Coherer ' made of iron. 

 This diminution of resistance was by no means general ; 

 his investigations revealed the astonishing fact that 

 potassium exhibited an effect which was diametrically 

 opposite, namely an increase of resistance. The receiver 

 made of potassium exhibited, moreover, a rapid and spon- 

 taneous recovery, requiring no tapping. It is quite evident 

 that an increase of resistance and automatic self-recovery 

 could on no account be due to the supposed fusion and 

 coherence of neighbouring particles by the induction-spark. 

 The response, positive or negative, is determined by the 

 chemical nature of the substance ; and the phenomonen 

 must therefore be one of molecular change. 



In arranging the elements in order of their increasing 

 atomic weights, the ' Electric Touch ' was found, as stated 

 before, to exhibit a remarkable periodicity, approximately 

 represented in the accompanying curve (Fig. i). 



Those above the horizontal line are positive, those below 

 negative, and others which cross the line more or less 

 neutral. Of the neutral substances, copper and silver 

 may be taken as typical. 



There are other interesting differences in the behaviours 

 of different metals. In some cases the induced change of 

 resistance under electric stress was not permanent, but the 

 substance completely recovered its original condition. It 

 was as if the molecules were put under strain by the 

 impressed stress. Electrically some were highly elastic, 

 their recovery being quick ; in other cases less elastic, and 

 the strained molecules remained in that condition, the 

 recovery being extremely slow. In such cases, however, 

 anything which caused molecular disturbance e.g. the 



